[llvm] [ELF] Attempt to set the OS when using 'makeTriple()' (PR #76992)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Is this testable?

There's not any code that tests it right now. I think it's annoying to test because it requires a unit test and a working binary. For non-NVPTX (because of PTX) we could probably just run the backend on an empty LLVM-IR file if nothing else if we really wanted to test it. It's a lot of effort however. 

There's a use of this in `llvm-objdump` but it doesn't print it anywhere we could check it doesn't seem.

https://github.com/llvm/llvm-project/pull/76992